The mechanism of bone marrow failure in acute leukemia is addressed in this research project. Using short-term tissue culture techniques, bone marrow cells from normal individuals or patients with acute leukemia in remission respond in a characteristic dose-response fashion to increasing concentrations of bone marrow colony stimulating factor. Patients with acute leukemia at presentation or relapse (even early relapse) have marrow cells which have colony stimulating factor dose response curves which are blunted compared to normals. This suggests some type of inhibition of normal marrow cell proliferation by the presence of leukemia cell clone. This work is designed to identify and characterize this inhibition using an in vitro assay of bone marrow cell growth. An assay of residual leukemia cells in marrow aliquots derived from these studies has been developed and has obvious importance in the management of patients with acute leukemia.